Mozilla is working on implementing a web installer for Firefox and the most recent Nightly builds can already be seen sporting it. The web installer has several advantages over the full installer currently being used by Firefox, smaller initial download, guaranteed to get the latest version and the right platform and so on.
The web installer, or stub installer as Mozilla calls it, will be a very small download and will be what people grab from the site, if they want to download the latest Firefox.
All of the actual Firefox files will be downloaded after the install process has started. This means that people are more likely to complete the install process, the small initial download means they won't get distracted while the installer arrives on their computer.
It also means it's less likely the download will fail. From there, people simply have to start the installer, click one button and they're done.